Ok so this is going to have a lot of unfavorable tropes so read at your own disclosure lol.

There was a manga where this adoptive father viewed his adopted son has his dead wife, but the son wanted the full love and attention of the father and to not be seen as the dead wife. However when the son goes to his high school there is a student there who has a major crush on him, turns out they're

siblings! . I think it ends off on ! the adoptive father dying in his house and the two siblings being together.

Hello!

I'm not sure if it's Ai o Kou Kemono. It's from 2011, could be considered old.

It has some similarities:

★Main character wants full attention from his adoptive father.
★Main character believes that his adoptive father thinks about his deceased mother.
★Classmate is in love with Main character and turns out they're brother's.
★Adoptive father dies.

But it has differences too:

★Adoptive father was actually his uncle and he had some kind of sister complex but loved his nephew.
★Main character and his brother don't end together, big brother left the house and main character starts to live with his real father after the burning in wich his uncle died.

Hope this helps!